The invention relates to the optimization of the use of data transmission resources in a data call, and particularly to the optimization of the use of traffic channels on the air interface of high speed data transmission services based on multichannel technology.
Modern mobile communication systems provide subscribers with both normal speech transmission and various data transmission functions. In mobile communication systems, the data transmission capacity available on the air interface is divided between several users by using a multiple access principle. The most common multiple access principles include time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA). In TDMA systems, communication over a radio path takes place on a time division basis in successive recurrent TDMA frames, each of which comprises several time slots. Time slots are mainly used for transferring control channels and traffic channels. Traffic channels are used for transmitting speech and data. In this application, data refers to any information conveyed in a digital telecommunication system. Such information may comprise digitized speech, inter-computer data communication, telefax data, short program-code segments etc. Control channels are used for signalling between a base transceiver station and mobile stations. An example of a TDMA radio system is the pan-European mobile system GSM (Global System for Mobile Communications).
In modern mobile communication systems, depending on the data transmission rate required, a traffic channel may comprise one subchannel (e.g. a TDMA time slot) or several subchannels (e.g. many TDMA time slots for a high speed data transmission). In the GSM system, for example, a high speed data service HSCSD (High Speed Circuit Switch Data) is defined, in which a traffic channel may comprise several subchannels. Channels and subchannels can be allocated symmetrically or asymmetrically. Correspondingly, a high speed data service has been planned, for example, for so-called third-generation mobile communication systems, such as the UMTS (Universal Mobile Telecommunication System) and the IMT-2000 (International Mobile Telecommunication 2000). Also in professional mobile radio systems, e.g. the TETRA (Terrestrial Trunked Radio), it is possible to allocate several subchannels to one connection. The user's data transmission rate on the air interface is affected by the number of subchannels and also the used channel coding method.
FIG. 1 shows one possible data transmission situation in the GSM mobile communication system. In the example of FIG. 1, a data transmission call is a call between two mobile stations. When a mobile station MS A makes a data transmission call to a mobile station MS B, a leg 1 is formed for the call, i.e. the connection, between the mobile station MS A and a serving mobile services switching centre MSC 1. Correspondingly, a leg 2 is formed for the same connection between the mobile station MS B and a serving mobile services switching centre MSC 2. A number of subchannels that is required by the data transmission rate are allocated to both legs 1 and 2. One leg is not aware of the situation of the other leg, although both legs were served by the same mobile services switching centre. As, for example, the data transmission rate of the call leg 1 on the air interface Air varies due to the upgrade, i.e. the increase in the number of subchannels, for instance, or due to the downgrade, i.e. the decrease in the number of subchannels, the leg 2 does not follow. As a result of upgrading the leg 1, the mobile station MS A may thus uselessly allocate subchannels from the air interface, which subchannels it cannot use because of the poorer data transmission rate of the leg 2. Correspondingly, as a result of downgrading the leg 1, the mobile station MS B may uselessly allocate subchannels from the air interface, which subchannels it cannot use due to the decreased data transmission rate of the leg 1. The situation remains the same, whether the mobile stations MS A and MS B are served by the same or a different mobile services switching centre.
A problem in the arrangement described above is that the air interface cannot be utilized in the most efficient way, because the information on the data transmission rate change of one leg is not conveyed to the other leg of the same connection. The efficient utilization of the radio spectrum is the main factor in planning and implementing mobile communication networks.
The inefficient use of traffic channels may present a problem also in a call between a mobile station and a fixed network. A fixed network part can provide (e.g. due to an autobauding handshaking of modems or the used fixed network protocol) a data rate much higher or much lower than requested at the call set-up stage. A problem may also be provisory, caused by the quality of the connection or network.
The inefficient use of traffic channels may present a problem in fixed network calls, too. For example, when data is transferred in a broadband network between two narrowband ISDN networks, several time slots can be allocated to the connection on both ISDN network sides, in which case the data transmission rate on different ISDN network sides is not necessarily the same and the resources of one side can be wasted.